The strong negative influence of the aging-related loss of muscle mass, strength and quality, i.e., sarcopenia, on impaired motor function in old age has become increasingly evident. Further, sarcopenia, which varies considerably between individuals, is a major determinant of healthy aging. The loss of muscle function associated with sarcopenia promotes a) inactivity, b) falls, and c) loss of confidence, which in turn can lead to physical dependency and associated medical and socio-economic problems. However, to date, the muscle wasting associated with aging has not been correlated with relevant genetic markers. The long-term goal of this project is to identify the genes influencing the aging-related loss of muscle mass and altered qualitative properties of skeletal muscle. Morphological and biochemical techniques will be used to assess aging- and gender-specific changes in skeletal muscle quantity and quality in a panel of recombinant strains, their progenitor strains, and the derived F2. Genetic analyses will subsequently be performed in order to locate chromosomal regions containing the quantitative trait loci (QTLs) which effect skeletal muscle quantity and quality. In future studies, results from QTL analysis with specific changes in muscle quantity and the quality of the molecular motor protein myosin, will permit both the sequencing of genes and the identification of specific products of those genes involved in the specific aging-related quantitative and qualitative changes in skeletal muscle, i.e., sarcopenia.